The present invention relates to a process for producing a calcined phosphate fertilizer, which is soluble in a citrate solution.
It is well known that the naturally occurring calcium phosphates which are found in the different parts of the world can be transferred into phosphate fertilizers, which are highly soluble in citrate solutions, by alkaline calcination decomposition. The principle procedure for the preparation of such alkaline high temperature phosphates is known already from the U.S. Pat. No. 1,799,882. This procedure comprises subjecting a mixture of raw natural phosphates, alkali carbonate and silicic acid to calcination in a rotary kiln at temperatures of above 1100.degree. C. Within the mixture which is to be calcinated, the proportions of the different components are chosen such that at least 1 mole of alkali oxide is included per mole of P.sub.2 O.sub.5. The added amount of silicic acid is portioned so that calcium orthosilicate may be formed therefrom by binding an equimolar amount of CaO from the tricalcium phosphate, which is present in the starting material and from the lime which is not bonded to phosphoric acid. The reaction which is assumingly taking place during such a process can be represented by the following equation: EQU 2 Ca.sub.3 (PO.sub.4).sub.2 + SiO.sub.2 + 2 Na.sub.2 CO.sub.3 = 2(Na.sub.2 O.sup.. 2 CaO.sup.. P.sub.2 O.sub.5) + 2 CaO.sup.. SiO.sub.2 + 2CO.sub.2
later studies revealed that in order to accomplish a complete alkaline high temperature decomposition of the naturally occurring raw phosphates which contain a major portion of fluoroapatite, it is advantageous if the molar ratio between the P.sub.2 O.sub.5, which is present in the raw phosphate and the alkali oxide which is used as a decomposing agent, is from 1:1 to 1:1.8. The decomposition is promoted by the presence of steam, whereby the latter can be provided by using fuels which are rich in hydrogen, especially fuel oil, as a combustible for producing the high temperature which is necessary for the decomposition process. For more than 50 years, calcium-sodium-silicophosphate, the phosphate component of which is plant-available on practically all types of cultivated soils, has been produced in this way using technical grade calcinated soda. Generally the quality, that is, the fertilizer activity of such high temperature phosphates is determined by the determination of the P.sub.2 O.sub.5 solubility in a test solution according to Petermann, in a neutral solution of ammonium citrate and in a 2% citric acid solution, whereby the first of these methods is preferred.
More recently, in addition to soda, aqueous solutions of alkali hydroxides have also been used commercially as decomposition agents. It has been found that alkali hydroxide solutions can also be used in a rotary kiln if a calcinatable agglomerate is prepared in a suitable manner from the raw phosphates, the necessary amount of silicic acid and the alkali hydroxide solution. According to the method which is disclosed in U.S. Pat. No. 3,985,537, it is even possible to introduce all the alkali hydroxide solutions, which are necessary for the decomposition, directly into the rotary kiln. Such a process can be performed economically if the alkali hydroxide solutions are available at low cost.
Ever since the alkaline decomposition process for producing alkaline calcinated phosphates was discovered, efforts have been made to lower the considerable costs with which the process is charged due to the use of relatively large amounts of alkaline decomposing agents. Thus, some years ago it has been tried to effect the decomposition of the raw phosphate by using naturally occurring alkali-containing silicates such as feldspar, phonolithe or leucite, together with fresh lime in such a way as to obtain an inexpensive fertilizer (see German Pat. No. 288,089). Yet the amounts of silicates, as well as the necessary amounts of lime which had to be applied, proved to be so high that the final products contained only about 12% of P.sub.2 O.sub.5 on the average. Furthermore, the P.sub.2 O.sub.5 solubilities in the test solution according to Petermann, were about 50%. Therefore, these products could not compare with other fertilizers.
Numerous efforts to use other natural products which contain alkali oxide generating compounds, as decomposing agents, gave unsatisfactory results. Using alkali chlorides and/or alkali sulfates did not prove satisfactory because thereby large amounts of highly corrosive contaminated hydrochloric or sulfurous acids occurred. Other decomposing agents have been proposed, such as alkali-containing waste liquids which are obtained from the decomposition of cellulose-containing materials like wood, molasses bottom products, or alkali-containing slags from the iron and steel industry. The use of any of these decomposing agents was troublesome and did not yield satisfactory results. Despite many proposals, no other alkali yielding materials have yet been found which do not influence the quality of the final product and which are less costly than those which were mentioned above. Because of the need for adding high amounts of other compounds, mostly lime, and of technical difficulties during the preparation and working up of the resulting products, these processes never became technically important.